Protection of structures, equipment, machines, and mechanisms made of ferrous metals from corrosion and effect exerted by aggressive media, an improvement of specifications of materials, including obtaining materials with prescribed properties, and development of resource-saving processing technologies are important scientific, technical and practical problems.
These problems can be solved by various methods, including applying powder coatings with widely usable gas flame-spray, electric arc, detonation and plasma methods.
The gas flame-spray method is based on gas combustion products used at 1000.degree. to 3000.degree. C., creation of a flow of these gases in which powder particles being applied are fused. A velocity of 50 to 100 m/s is imparted to said particles, and the surface is treated with the gas and powder flow containing the fused particles. This treatment results in a coating. The low values of velocity and temperature of the applied particles substantially limit application of this method.
The explosive method eliminates these disadvantages in part, according to which the energy of detonating gases at 2000.degree. to 3500.degree. C. is used owing to which fact the velocity of the particles is substantially increased up to 400 to 700 m/s and their temperature is increased up to 2000.degree. to 3500.degree. C. to ensure application of coatings of powders of metals, alloys, and dielectrics. This method is highly disadvantageous in low productivity explained by the impact acceleration process of deposition: a resulting shock wave and a gas flow following it cause a high level of a thermal and dynamic pulse effect produced upon the product and also of acousting noise which restricts the possibilities of application of this method.
The most promising is a method of plasma deposition consisting in application of a powder coating to the surface of a product with a high-temperature gas jet (5000.degree. to 3000.degree. C.).
Known in the art is a method for applying coatings to the surface of a product whose material is selected from the group consisting of metals, alloys, and dielectrics, said method comprising introducing into a gas flow a powder of the material selected from the group consisting of metals, alloys, their mechanical mixtures or dielectrics to form a gas and powder mixture to be directed onto the surface of the product (the book V. V. Kudinov, V. M. Ivanov. Nanesenie Plazmoi Tugoplavkikh Pokryty /Application of Refractory Coatings with Plasma/. Mashinostroenie Publishing House, Moscow. 1981, pp.9 to 14).
The prior art method is characterized in that powder particles of a size of from 40 to 100 .mu.m are introduced into a high-temperature gas flow (5000.degree. to 3000.degree. C.) in the form of a plasma jet. Said powder particles are heated to the melting point or higher, the powder particles are accelerated by the plasma jet gas flow and directed to the surface being coated. Upon impingement, the powder particles interact with the surface of a product thus forming the coating. In the prior art method, the powder particles are accelerated by a high-temperature plasma jet and transferred, in molten state, to the product being coated; as a result, the high-temperature jet runs in the product to exert a thermal and dynamic effect upon its surface, i.e., causes local heating, oxidation and thermal deformations. For instance thin-walled products are heated up to 550.degree. C., oxidized and twisted while the coating peels off.
The high-temperature jet flowing into the surface of a product intensifies chemical and thermal processes, causes phase transformations and appearance of oversaturated and non-stoichiometric structures, and hence, the structural changes in the material. Also the high level of a thermal effect on the coating results in hardening heated melts and gas liberation during crystallization which bring about the formation of evolved porosity and appearance of microcracks, i.e., impairs specifications of the coating.
It is known that, with an increase in the temperature of a plasma jet, plasma density in comparison with gas density under normal conditions linearly decreases, i.e., at 1000.degree. C., density of the jet becomes scores of times a factor that results in a lower resistance coefficient of particles. To sum up given a plasma jet velocity of 1000 to 2000 m/s (which is about equal to, or slightly below then, the sonic velocity), the particles are accelerated up to 50 to 200 m/s (even up to 350 m/s at best), i.e., the process of acceleration is not efficient enough.
As is known with a decrease in a size of powder particles heating, melting, and overheated thereof in a plasma jet are enhanced. As a result, the, fine fractions of powder of a size from 1 to 10 .mu.m are heated to a temperature above the melting point, and their material intensively evaporates. For this reason, the plasma deposition of particles having a size below 20 to 40 .mu.m causes great difficulties and particles of a size from 40 to 100 .mu.m are normally used for this purpose.
It should also be noted that the prior art method makes use of plasma jets of energy-consuming diatomic gases which call for application of high power which explains stringent requirements imposed upon the structure of apparatuses. It is only natural that limitations of the method of deposition on small-size objects are rather essential and can be eliminated through the complete removal of energy applied by cooling or providing a dynamic vacuum, i.e., by evacuation of high-temperature gases which requires high power consumption.
Therefore, the prior art method has the following disadvantages: the high level of thermal and dynamic effect on the surface being coated; substantial changes in properties of the material being applied during the coating application, such as electrical conductance, heat conductance, and the like; changes in the structure of material as a result of phase transformations and appearance of oversaturated structures following from the chemical and thermal effect of the plasma jet and the hardening of overheated melts; ineffective acceleration of powder particles resulting from low density of plasma; intensive evaporation of fine powder fractions of a size from 1 to 10 .mu.m; stringent requirements imposed upon the structure of apparatus in view of hightemperature processes of the prior art method.
Known in the art is an apparatus for carrying out the prior art method for applying coatings to the surface of a product, comprising a metering feeder having a casing accommodating a hopper for a powder communicating with a means for metering the powder formed as a drum having depressions in its cylindrical surface, a mixing chamber and also provided with a nozzle for accelerating powder particles communicating with the mixing chamber, a source of compressed gas, and a means connected thereto to supply the compressed gas to the mixing chamber (in the book V. V. Kudinov, V. M. Ivanov, Nanesenie Plazmoi Tugoplavkikh Pokryty /Application of Refractory Coatings with Plasma/. Mashinostroenie Publishing House, Moscow. 1981, pp.20 to 21, FIG. 11; p.26, FIG. 13).
The prior art apparatus is characterized by a plasma sprayer (plasmotron), comprising a cylindrical (subsonic) nozzle having passages for supplying a plasma-forming gas and water for cooling thermally stressed units of the plasma sprayer (namely, the nozzle) in which refractory materials are used. Powder particles are introduced from the metering feeder at the edge of the nozzle.
Since energy for forming plasma jet is applied in the form of an arc in the passage of a plasmotron nozzle, the nozzle is subjected to intensive electric erosion and high-temperature exposure. As a result, a rapid erosion wear of the nozzle occurs, the service life of which is 15 to 20 hours. With the sophisticated construction and use of refractory materials and water cooling service life can be prolonged to 100 hours.
The introduction of the particles at the edge of a nozzle and erosion of the inner duct of the nozzle lower the efficiency of acceleration of the powder particles. Thus, in combination with a low density of plasma, the prior art apparatus ensures a velocity of powder particles of up to 300 m/s with a gas escape velocity of up to 1000 m/s.
As a result of the powder getting into the space between moving parts of the metering feeder (e.g., between the drum and casing), the drum tends to be jammed.
Therefore, the prior art apparatus has the following disadvantages: short service life which is mainly determined by the service life of a nozzle of 15 to 100 hours and which is associated with the high density of a heat flux in the direction towards the plasmotron nozzle and erosion of the electrodes a factor that makes one to use expensive, refractory, and erosion-resistant materials; the inefficient acceleration of the deposited particles because the nozzle design is not optimal and is subjected to changes entailed by the electrical erosion of the inner duct; unreliable operation of the metering feeder of is the drum type which is caused by the powder getting into the space between the moving parts thus causing their jamming.